Can I spend the money that the Charter School gives me on Christian curriculum?

No.

Many Charter Schools, such as Inspire or Valley Oaks, provide you
"free" money
to spend on curriculum. You cannot legally spend any money that they give you on
Christian Curriculum. If they don't give you money, but allow you to
order your curriculum through them, they are not legally allowed to
provide you with Christian curriculum.

Charter Schools are public schools, and the money that the Charter
Schools gives you is tax payer (public) money. The California Constitution
(Article IX. Section 8) states:

No public money shall ever be appropriated for the support
of any sectarian or denominational school, or any school not under
the exclusive control of the officers of the public schools; nor
shall any sectarian or denominational doctrine be taught, or
instruction thereon be permitted, directly or indirectly, in any of
the common schools of this State.

Therefore, it is against the law for you to use the tax payer money
that the Charter School is providing for any form of Christian
literature.

But they let me buy Christian Curriculum now

It doesn't matter. It is against California law! If the charter
school provides you with Christian curriculum, they are breaking the
law. If you buy Christian curriculum with money provided to you by a
charter school, you are breaking the law.

What are your reasons for homeschooling? Are you really trying to
give your children a Christian education? You are teaching them that
it is ok to break the law, as long as you get some "free stuff."